Fighting in northern Syria between Turkey-backed Syrian opposition forces and militants from the Islamic state group has killed at least 15 rebels as the opposition tries to push toward a town that carries symbolic importance for the extremists, an activist group and Turkish officials said Monday.

The deaths toll among the rebels is the highest since Turkey sent troops and tanks into Syria in August to help rebels re-take IS strongholds near the border and curb the advance of a Syrian Kurdish militia, which Ankara accuses of links with Turkey's outlawed Kurdish separatists.

Turkish military officials said 15 Syrian opposition fighters were killed and about 35 were wounded in the fighting, which seeks to capture seven residential districts south of the town of al-Rai. According to an emailed statement, "intense'' clashes had taken place in the regions of Boztepe, Hardanah and Turkmen Bari.

The statement said the casualties occurred over the last 24 hours. The Turkish officials spoke on condition of anonymity in line with regulations.

The Britain-based Syrian Observatory for Human Rights said 21 rebels were killed and more than two dozen wounded, adding that many of the casualties were due to land mines and booby-traps planted by the extremists near the village Turkmen Bari.

The Observatory, which relies on a network of activists across Syria, said the Turkey-backed rebel forces are trying to reach the town of Dabiq. IS propaganda claims the town is going to be the location of an epic battle with invading Christian forces in which Muslims will win.

IS has been preparing for the battle in Dabiq for weeks, planting mines and explosives and sending some of its most experienced fighters to defend the town, which has been in IS hands since August 2014. The extremists have named their online magazine after the town.

Meanwhile, Russian and Syrian government warplanes carried out more airstrikes around the country, mostly in eastern rebel-held neighborhoods of the northern city of Aleppo, Syria's largest urban center and once its commercial hub.

Also Monday, a medical relief group and the Observatory said airstrikes have damaged and put of service one of Syria's most secure hospitals, which had been dug into a mountain.

The International Union of Medical Care and Relief Organizations, or UOSSM, said the Dr. Hasan Al-Araj — also known as "Cave Hospital'' and located in the central province of Hama — was struck twice on Sunday.

The Observatory said Russian warplanes carried out the attacks that hit the hospital, near the central village of Kfar Zeita, adding that it's one of the largest hospitals in rebel-held parts of the country. UOSSM said there were minor injuries from the attack.

Dr. Abdallah Darwish, the hospital's director and health care chief in Hama province, was quoted in the UOSSM statement as saying that the hospital was likely struck by "bunker buster'' missiles as it is "well-fortified in a cave and impervious to previous attacks.''

The bomb completely destroyed the hospital's emergency department and caused major damage throughout the hospital, he said.

Syrian and Russian warplanes have been blamed for a series of attacks that have damaged hospitals and clinics in rebel-held parts of Syria, mostly in the northern city Aleppo.

Dr. Khaula Sawah, CEO of UOSSM USA, said the situation in Syria is becoming "more and more dire as every day passes.''

"These vicious and atrocious campaigns are literally choking the life out of civilians, they are deplorable and unacceptable,'' she said. "We demand the international community and all responsible parties put an immediate stop to this before it is too late.''